It is well known that permanganate in acidic solution can oxidize oxalate ions (or oxalic acid). The reaction produces $\ce{Mn^2+}$, which catalyzes this very reaction.

$$\ce{2MnO4^- + 16H^+ + 5C2O4^2- -> 2Mn^2+ + 8H2O + 10CO2}$$

As a part of a demonstration on chemical kinetics, I want to inhibit this autocatalysis somehow. (I intend to perform three reactions: one with normal autocatalysis, one with $\ce{Mn^2+}$ added, which will run faster, and one preventing the $\ce{Mn^2+}$ from catalyzing the reaction.)

At first I thought I might use EDTA to complex the manganese ions. Alas, it didn't help. Maybe the environment is too acidic. (The reaction was actually faster with EDTA, I don't know why.)

Perhaps I should try a different chelating agent? Perhaps I could precipitate the $\ce{Mn^2+}$ ions? What do you suggest?


1 Answer 1


At first the reaction is slow. But when we heat up, the reaction gets catalyzed due to formation of $\ce{Mn^2+}$ ions. It happens due to the fact that $\ce{Mn^2+}$ gets oxidised to $\ce{Mn^3+}$ ions. (Source).

$$\ce{4Mn^2+ + MnO4- + 8H+ -> 5Mn^3+ + 4H2O}$$

So, we have to inhibit the formation of $\ce{Mn^3+}$ ions so as to inhibit the autocatalysis reaction. For this, we have to use phosphonate and carboxylate based chelating agent that solubilize (hydr)oxide-bound $\ce{Mn(III)}$ . The following is some relevant information from the site:

phosphonate-based chelators that solubilize (hydr)oxide-bound Mn(III) via ligand-promoted and/or reductive dissolution at circum-neutral pHs.

pyrophosphoric acid (PP), methylenediphosphonic acid (MDP), and phosphonoacetic acid (PAA), ligand-promoted dissolution is predominant: from pH 6−8, initial dissolution rates and plateau concentrations for dissolved Mn(III) decrease in the order PP > MDP > PAA, and at pH 5, MDP reacts equally well (with birnessite) or more efficiently (with manganite) than PP, and PAA remains the least reactive chelator. For manganite reacting with an excess concentration of aminophosphonate/carboxylate-based chelators, the aminophosphonate-containing iminodimethylenephosphonic acid and glyphosate yield appreciable amounts of dissolved Mn(III), but the aminocarboxylate-based methyliminodiacetic acid yields solely dissolved Mn(II) via Mn(III) reduction.


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